Discovery of new property in commonly used plastic leads to invention

New memory device could offer smaller, simpler way to archive data

By Steven Schultz

Princeton NJ -- Engineers at Princeton and Hewlett-Packard have invented a combination of materials that could lead to cheap and super-compact electronic memory devices for archiving digital images or other data.

The invention could result in a single-use memory card that permanently stores data and is faster and easier to use than a compact disk. The device could be very small because it would not involve moving parts such as the laser and motor drive required by CDs.

The researchers, who published a description of the device in the Nov. 13 issue of Nature, achieved the result by discovering a previously unrecognized property of a commonly used conductive polymer plastic coating. Their memory device combines this polymer, which is inexpensive and easy to produce, with very thin-film, silicon-based electronics.

"We are hybridizing," said Princeton Professor of Electrical Engineering Stephen Forrest, who led the research group. "We are making a device that is organic (the plastic polymer) and inorganic (the thin-film silicon) at the same time."

As a result, the device would be like a CD in that writing data onto it makes permanent physical changes in the plastic and can be done only once. But it also would be like a conventional electronic memory chip because it would plug directly into an electronic circuit and would have no moving parts. "The device could probably be made cheaply enough that one-time use would be the best way to go," Forrest said.

The research was done in Forrest's lab by former postdoctoral researcher Sven Möller, who is now at HP in Corvallis, Ore. Craig Perlov, Warren Jackson and Carl Taussig, scientists at HP Labs in Palo Alto, Calif., are also co-authors of the Nature paper.

Möller made the basic discovery behind the device by experimenting with polymer material called PEDOT, which is clear and conducts electricity. It has been used for years as an antistatic coating on photographic film, and more recently as an electrical contact on video displays that require light to pass through the circuitry. Möller found that PEDOT conducts electricity at low voltages, but permanently loses its conductivity when exposed to higher voltages (and thus higher currents), making it act like a fuse or circuit breaker.

This finding led the researchers to use PEDOT as a way of storing digital information. Digital images and all computerized data are stored as numbers that are written as long strings of ones and zeroes. A PEDOT-based memory device would have a grid of circuits in which all the connections contain a PEDOT fuse. A high voltage could be applied to any of the contact points, blowing that particular fuse and leaving a mix of working and non-working circuits. These open or closed connections would represent zeros and ones and would become permanently encoded in the device. A blown fuse would block current and be read as a zero, while an unblown one would let current pass and act as a one.

This grid of memory circuits could be made so small that, based on the test junctions the researchers made, 1 million bits of information could fit in a square millimeter of paper-thin material. If formed as a block, the device could store more than one gigabyte of information, or about 1,000 high-quality images, in one cubic centimeter, which is about the size of a fingertip.

Developing the invention into a commercially viable product would require additional work on creating a large-scale manufacturing process and ensuring compatibility with existing electronic hardware, a process that might take as little as five years, Forrest said.

Research that combines expertise on both "hard" and "soft" materials, such as the silicon and polymer materials in Forrest's memory device, represents a major strength at Princeton and is the focus of the newly established Princeton Institute for the Science and Technology of Materials. The institute includes scientists and engineers from a wide range of disciplines and seeks to combine basic science and commercial partnerships.

Forrest, the James McDonnell Distinguished University Professor of Electrical Engineering, has been a faculty member at Princeton since 1992. His work in organic electronics has generated insights into the fundamental properties of photonic materials and spawned a range of commercially applicable discoveries, including technology for flat-panel video displays (see related article at left), electronic sensors and solar energy devices.

Funding for Forrest's research on memory devices came in part from HP as well as from the National Science Foundation through a long-term grant that funds a Materials Research Science and Engineering Center at Princeton. The University has filed for a patent on the invention. HP has an option to license rights to the technology.

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